The present invention relates to a cathode ray tube device, and it relates, in particular, to a structure near an electron gun and a velocity modulation coil.
FIG. 3 is a lateral cross-sectional view showing a cathode ray tube device. As shown in FIG. 3, the cathode ray tube device includes a cathode ray tube, a deflection yoke 5, a convergence yoke 7 and velocity modulation coils 6. The cathode ray tube has a front panel 1 whose inner surface is provided with a phosphor screen 8, a funnel 2 and an electron gun 4 provided inside a neck portion 3 of the funnel 2. The deflection yoke 5 has horizontal deflection coils and vertical deflection coils that are mounted on an outer surface of the funnel 2 and positioned on the side of the front panel 1 with respect to the electron gun 4. The convergence yoke 7 is provided on an outer surface of the neck portion 3.
FIG. 11 is a lateral cross-sectional view of the neck portion 3. The electron gun 4 (shown not as a cross-sectional view) has a structure in which a cathode 21, a control electrode (a G1 electrode) 22, an accelerating electrode (a G2 electrode) 23, a focusing electrode (a G3 electrode) 24 and an anode 25 having a G4 electrode 26 and a top unit 27 are arranged sequentially. The top unit 27 is a cup-shaped member having a cylindrical portion and a bottom portion that is provided with an electron beam passing hole. Until electron beams 9 (shown in FIG. 3) emitted from the cathode 21 reach the phosphor screen 8 formed on the inner surface of the front panel 1, their paths are deflected by an ac magnetic field generated by the deflection yoke 5, the velocity modulation coils 6 (which are not true to life in FIG. 11 for the sake of convenience, but actually are formed as shown in FIG. 2) and the convergence yoke 7. The deflection yoke 5 includes horizontal deflection coils 51 for deflecting the paths horizontally and vertical deflection coils 52 for deflecting the paths vertically and is mounted on a cone portion of the funnel 2. The deflection yoke 5 generates the ac magnetic field so as to deflect the paths of the electron beams, thereby scanning the phosphor screen with the electron beams. The convergence yoke 7 is mounted outside the neck portion 3 and focuses the three electron beams on one point by its magnetic field.
In a current advanced display technology, the magnetic field is modulated by the velocity modulation coils 6 so as to perform what is called a velocity modulation of electron beams, thereby improving the focus performance (see JP 10(1998)-74465 A). The velocity modulation coils 6 are each arranged between the convergence yoke 7 and the neck portion 3 and at a position where the G3 electrode 24 and the G4 electrode 26 are located. The velocity modulation coils 6 generate an ac magnetic field 28 (shown as xe2x80x9ca barrel shapexe2x80x9d with dashed lines) so as to modulate a scanning velocity of the electron beams, thereby realizing a high-brightness portion and a low-brightness portion on the phosphor screen, thus achieving a sharp image.
The frequency of the ac magnetic field 28 for modulating the electron beams is of the order of a megahertz, as high as a video frequency. Therefore, when the velocity modulation coils 6 are provided at the position shown in FIG. 11, the ac magnetic field 28 is attenuated by the G3 electrode 24 and the G4 electrode 26, which are formed of a metallic material such as stainless steel, causing a problem in that the electron beams cannot be modulated in a desired manner. In other words, the ac magnetic field 28 generates eddy currents in the G3 electrode 24 and the G4 electrode 26, causing a loss of the ac magnetic field 28.
Conventionally, it has been suggested that an electrode formed by deep-drawing should be divided into several parts, which are then spaced away from each other so as to improve magnetic permeability (see JP 8(1996)-115684 A). However, when the distance between the electrodes in the electron gun are designed to be great, an electric potential permeating into the neck portion separates the three electron beams that have been focused on one point on the phosphor screen, causing a problem in practical use. There also have been problems in that an assembling accuracy lowers, costs increase, and the magnetic permeability cannot be improved considerably because the size of each component should not be reduced too much in order to maintain a mechanical strength of each of the divided electrodes.
In addition, it is suggested in JP 5(1993)-347131 A that velocity modulation coils should be provided to overlap horizontal deflection coils, thus forming a portion in which an electrode of an electron gun and the velocity modulation coil do not overlap each other, thereby improving a modulation sensitivity of the velocity modulation coil. In this case, the frequency of an ac magnetic field from the velocity modulation coils is of the order of a megahertz and higher than the video frequency, and therefore, this ac magnetic field interferes with the magnetic field from the horizontal deflection coils, thus deteriorating signals of a television device. This leads to a poor image quality, becoming inappropriate for a practical use.
The present invention has been made in order to solve the problems described above, and it is an object of the present invention to provide a cathode ray tube device that can achieve a desired modulation effect on electron beams without blocking permeation of a velocity modulation magnetic field from an external side of a cathode ray tube.
A first cathode ray tube device of the present invention includes a cathode ray tube including a front panel, a funnel and an electron gun that is provided inside a neck portion of the funnel, a deflection yoke including a horizontal deflection coil and a vertical deflection coil that are mounted on an outer surface of the funnel and positioned on a side of the front panel with respect to the electron gun, and a velocity modulation coil that is mounted on an outer surface of the neck portion. An end of the velocity modulation coil on the side of the front panel is positioned on a side of the electron gun with respect to an end of the horizontal deflection coil on the side of the electron gun and is positioned on the side of the front panel with respect to an end of the electron gun on the side of the front panel.
With the above structure, since the horizontal deflection coil of the deflection yoke and the velocity modulation coil do not overlap in a direction perpendicular to a tube axis of the cathode ray tube, no interference from these coils deteriorates signals of a television device so as to cause a poor image quality. Also, because at least a part of the velocity modulation coil on the side of the front panel does not overlap a screen-side end of an electrode of the electron gun in the direction perpendicular to the tube axis of the cathode ray tube, it is possible to reduce a loss of an ac magnetic field from the velocity modulation coil owing to eddy currents, thereby achieving a desired modulation effect on electron beams.
It also is preferable that a distance along a tube axis direction of the cathode ray tube between the end of the velocity modulation coil on the side of the front panel and the end of the electron gun on the side of the front panel is at least 10% of a length of the velocity modulation coil along the tube axis direction. With this structure, it is possible to reduce the loss of the ac magnetic field from the velocity modulation coil owing to the eddy currents, thereby achieving a desired modulation effect on electron beams.
Furthermore, it is preferable that a distance along a tube axis direction of the cathode ray tube between the end of the velocity modulation coil on the side of the front panel and the end of the electron gun on the side of the front panel is at least 1 mm and not greater than 10 mm. With this structure, it is possible to reduce the loss of the ac magnetic field from the velocity modulation coil owing to the eddy currents, thereby achieving a desired modulation effect on electron beams.
Moreover, it is preferable that a component at the end of the electron gun on the side of the front panel includes a cylindrical component, and that the cylindrical component has a length along a tube axis direction of 10% to 30% of an outer diameter of the cylindrical component. With this structure, it is possible to prevent problems such as a strength decrease, a decrease in the insulation between an electrically conductive film applied onto an inner surface of the neck portion of the cathode ray tube and a G3 electrode, and an adverse effect of an electric potential of the electrically conductive film on a main lens while maintaining a short top unit of the electron gun.
It also is preferable that a cylindrical portion of the cylindrical component is provided with an opening. With this structure, providing the opening decreases a total amount of the eddy currents, thus achieving a sufficient loss-reduction effect.
Furthermore, it is preferable that a front-panel-side end of a cylindrical portion of the cylindrical component is provided with a notch. With this structure, providing the notch decreases a total amount of the eddy currents, thus achieving a sufficient loss-reduction effect.
A second cathode ray tube device of the present invention includes a cathode ray tube including a front panel, a funnel and an electron gun that is provided inside a neck portion of the funnel, a deflection yoke including a horizontal deflection coil and a vertical deflection coil that are mounted on an outer surface of the funnel and positioned on a side of the front panel with respect to the electron gun, and a velocity modulation coil that is mounted on an outer surface of the neck portion. A component at an end of the electron gun on the side of the front panel includes a cylindrical portion and a coil-shaped portion that is provided on the side of the front panel with respect to the cylindrical portion. An end of the velocity modulation coil on the side of the front panel is positioned on a side of the electron gun with respect to an end of the horizontal deflection coil on the side of the electron gun and is positioned on the side of the front panel with respect to an end of the cylindrical portion of the electron gun on the side of the front panel.
With the above structure, since reducing the generation of the eddy currents in the coil-shaped portion allows the velocity modulation magnetic field to permeate through the coil-shaped portion efficiently, it is possible to achieve a desired velocity modulation effect over a wide range of frequencies.
It also is preferable that a space between adjacent wires of the coil-shaped portion is not greater than 2.5 mm. With this structure, since the velocity modulation magnetic field can permeate through the coil-shaped portion efficiently, it is possible to achieve a desired velocity modulation effect over a wide range of frequencies.
Furthermore, it is preferable that adjacent wires of the coil-shaped portion are in contact with each other. With this structure, since the generation of the eddy currents is smaller than in the case of a cylindrical top unit, which allows the velocity modulation magnetic field to permeate through the coil-shaped portion more easily, it is possible to achieve a desired velocity modulation effect over a wide range of frequencies.